This invention relates to an electromagnetic actuator utilizing a permanent magnet formed into a hollow cylindrical configuration having a thin wall and driven in its axial direction under application of a unipolar magnetic field.
Heretofore, a voice coil type actuator used in a dynamic speaker has been widely known as an example of the aforementioned actuator.
However, the driving force of the actuator which is otherwise termed a voice coil motor is expressed as follows: EQU B.sub.f .multidot.l.multidot.i.sub.a
wherein:
B.sub.f represents a magnetic field applied to the moving coil, PA0 l represents an effective length of the moving coil, and PA0 i.sub.a represents an electric current flowing through the moving coil.
In order to increase the driving force of the moving coil having a predetermined effective length l, either one of the electric current i.sub.a and the magnetic field B.sub.f must be increased. Since an increase in the electric current i.sub.a entails an increase in the cross-sectional area of the conductor, and any attempt intensifying the magnetic field B.sub.f inevitably increases the size of the permanent magnet, the driving force of the conventional voice coil type actuator has been restricted to a comparatively small value.